In a manufacturing process of a semiconductor device such as IC or LSI, for example, when a contact hole is formed, an etching is performed on a silicon nitride film (SiNx film) through a mask of a silicon oxide film (SiO2 film).
Generally, such an etching is performed in a plasma etching apparatus. In the plasma etching apparatus, for example, in a processing chamber, after a substrate including a silicon oxide film and a silicon nitride film formed on top thereof is mounted on a lower electrode, high frequency powers are applied to an upper electrode and the lower electrode and a processing gas is introduced into the processing chamber to generate a plasma. The silicon nitride film is etched by the plasma action.
Conventionally, when etching the silicon nitride film, a gaseous mixture containing CHF3 gas, O2 gas and Ar gas has been used as a processing gas. However, in case of using the gaseous mixture, a selectivity of the silicon nitride film to the silicon oxide film (an etching rate of the silicon nitride film/an etching rate of the silicon oxide film) is low, the ratio being approximately 2.0.
Therefore, in order to increase the selectivity of the silicon nitride film to the silicon oxide film, there has been proposed a technique of using a gaseous mixture containing CH2F2 gas, O2 gas and Ar gas as a processing gas (see, e.g., Reference 1).
[Reference 1] Domestic Re-Publication of PCT patent application WO 98/16950
However, in case of using the gaseous mixture containing CH2F2 gas, O2 gas and Ar gas, although the selectivity increases, there may occur a bowing phenomenon, wherein sidewalls of grooves on the silicon nitride film are etched to be concaved, or reaction products may become attached to the sidewalls of the grooves to form deposits thereon, causing the silicon nitride film to be etched into an improper shape. If the silicon nitride film is etched into an improper shape, for example, a fine contact hole having a high dimensional accuracy may not form, which, in turn, will likely make it difficult to form high performance semiconductor devices.